There has been proposed a technique of producing an extremely thin polarizing film by forming a polyvinyl alcohol-based resin (hereinafter referred to as “PVA-based resin”) layer on a thermoplastic resin substrate, and stretching and dyeing the PVA-based resin layer formed on the resin substrate, integrally with the resin substrate.
For example, JP 4279944B (Patent Document 1) describes a method of producing a polarizing plate. This method comprises: forming a PVA-based resin layer on one of opposite surfaces of a thermoplastic resin film to have a thickness of 6 μm to 30 μm by a coating process; stretching them at a stretching ratio of 2 to 5 times to transform the PVA-based resin layer into a transparent coating element layer, thereby forming a two-layered composite film composed of the thermoplastic resin film and the transparent coating element layer; laminating an optical transparent resin film layer onto the side of the transparent coating element layer of the two-layered composite film through a bonding agent; peeling and removing the thermoplastic resin film; and dyeing and stabilizing the transparent coating element layer to form a polarizing element layer. The polarizing plate obtained by this method has a two-layer structure composed of the optical transparent resin film layer and the polarizing element layer. According to the description of the Patent Document 1, the polarizing element has a thickness of 2 to 4 μm.
JP 2001-343521A (Patent Document 2) and JP 2003-043257A (Patent Document 3) also describe a method of producing a polarizing plate. This method comprises: uniaxially stretching a laminate composed of a thermoplastic resin substrate and a PVA-based resin layer applied and formed on the substrate, at a given temperature, for example, at a temperature of 70° C. to 120° C., in the case where the substrate is made of a non-crystallizable polyester resin; and dyeing the PVA type resin layer molecularly oriented by the stretching, to cause a dichroic material to be absorbed in the PVA type resin layer (i.e., cause the PVA type resin layer to be impregnated with a dichroic material). The Patent Document 2 includes a description that the uniaxial stretching may be either one of longitudinal uniaxial stretching and transverse uniaxial stretching. On the other hand, the method described in the Patent Document 3 is configured to subject the laminate to transverse uniaxial stretching, and, during or after the transverse uniaxial stretching, cause the laminate to shrink in length in a direction perpendicular to the stretching direction by a specific value. In both of the Patent Documents 2 and 3, a thickness of an obtainable polarizing film is described as being in the range of 1 to 1.6 μm.
JP 4751486B (Patent Document 4) describes a method of producing a polarizing film made of a PVA-based resin having a molecularly-oriented dichroic material and formed on a resin substrate to have a thickness of 7 μm or less. This method comprises: forming a PVA-based resin layer on a resin substrate; dyeing the PVA-based resin layer to cause a dichroic material to be absorbed in the PVA-based resin layer; stretching the PVA-based resin layer having the dichroic material absorbed therein, in a boric acid aqueous solution, integrally with the resin substrate, so as to allow a total stretching ratio to become 5 times or more an original length thereof. The method described in the Patent Document 4 makes it possible to produce a thin polarizing film having excellent optical characteristics.
JP 4751481B (Patent Document 5) describes a method of producing a continuous web of a polarizing film comprising a PVA-based resin having a molecularly-oriented dichroic material. This method comprises subjecting a laminate which comprises a non-crystallizable ester-based thermoplastic resin substrate and a PVA-based resin layer formed on the substrate, to a 2-stage stretching process consisting of preliminary in-air stretching and in-boric-acid-solution stretching. The method described in the Patent Document 5 makes it possible to obtain a polarizing film having a thickness of 10 μm or less and exhibiting high-level optical characteristics satisfying the following conditions: T≥42.3 and P≥99.9, where T is a single transmittance, and P is a polarization rate.
As above, in recent years, there have been proposed a large number of techniques for producing a polarizing film having a small thickness and exhibiting high-level polarizing characteristics, by forming a PVA-based resin layer on a thermoplastic resin substrate, and stretching and dyeing the PVA-based resin layer integrally with the substrate.
The polarizing film produced in the above manner is usable as a component or material for an optical display device, without any change, i.e., directly together with the thermoplastic resin substrate, or after attaching an optical film, such as a polarizer protection film or a phase difference film, to one surface of the polarizing film on a side opposite to the other surface as a contact surface with the thermoplastic resin substrate during production thereof, to thereby provide various functions thereto, and optionally peeling the thermoplastic resin substrate.